Journal of Animal Blackwell Publishing Ltd Ecology 2006 Contrasting cascades: insectivorous birds increase pine but 75, 350–357 not parasitic mistletoe growth

KAILEN A. MOONEY* and YAN B. LINHART University of Colorado, Department Ecology and Evolutionary Biology, Boulder, CO 80309-0334, USA

Summary 1. Intraguild predation occurs when top predators feed upon both intermediate predators and herbivores. Intraguild predators may thus have little net impact on her- bivore abundance. Variation among communities in the strength of trophic cascades (the indirect effects of predators on plants) may be due to differing frequencies of intraguild predation. Less is known about the influence of variation within communities in predator–predator interactions upon trophic cascade strength. 2. We compared the effects of a single predator community between two sympatric plants and two herbivore guilds. We excluded insectivorous birds with cages from ponderosa pine Pinus ponderosa trees parasitized by dwarf mistletoe Arceuthobium vaginatum. For 3 years we monitored caged and control trees for predatory arthropods that moved between the two plants, foliage-feeding caterpillars and sap-feeding hemi- pterans that were host-specific, and plant damage and growth. 3. Excluding birds increased the abundance of ant-tended aphids on pine and resulted in an 11% reduction in pine woody growth. Mutualist ants protected pine-feeding aphids from predatory arthropods, allowing aphid populations to burgeon in cages even though predatory arthropods also increased in cages. By protecting pine-feeding aphids from predatory arthropods but not birds, mutualist ants created a three-tiered linear food chain where bird effects cascaded to pine growth via aphids. 4. In contrast to the results for tended aphids on pine, bird exclusion had no net effects on untended pine herbivores, the proportion of pine foliage damaged by pine-feeding caterpillars, or the proportion of mistletoe plants damaged by mistletoe-feeding cater- pillars. These results suggest that arthropod predators, which were more abundant in cages as compared with control trees, compensated for bird predation of untended pine and mistletoe herbivores. 5. These contrasting effects of bird exclusion support food web theory: where birds were connected to pine by a linear food chain, a trophic cascade occurred. Where birds fed as intraguild predators, the reticulate food webs linking birds to pine and mistletoe resulted in no net effects on herbivores or plant biomass. Our study shows that this variation in food web structure occurred between sympatric plants and within plants between differing herbivore guilds. Key-words: bird exclusion, community ecology, effect size, food web structure, indirect effect, intraguild predation, predator exclusion, top-down, tri-trophic interaction Journal of Animal Ecology (2006) 75, 350–357 doi: 10.1111/j.1365-2656.2006.01054.x

progressed to the task of determining when and where Introduction top-down control is likely to be important (Matson & The long-standing debate over whether terrestrial plants Hunter 1992; Schmitz, Hamback & Beckerman 2000; are protected from herbivores by predators has now Halaj & Wise 2001). Trophic cascade theory is predi- cated on the assumption of neatly tiered trophic levels © 2006 The Authors. Correspondence and present address: Cornell University, where organisms interact through linear food chains Journal compilation Department Ecology and Evolutionary Biology. Corson (Hairston, Smith & Slobodkin 1960; Polis & Strong © 2006 British Hall, Ithaca, NY 14853, USA. Tel.: 607 255–8050. 1996). Yet when predators prey upon both herbivores Ecological Society E-mail: [email protected] and other predators, the resultant network of direct

351 and indirect interactions causes predator effects to parasites that tap into host xylem and phloem to obtain Contrasting attenuate before herbivore abundance and plant growth water, minerals and photosynthates (Hawksworth & cascades from birds are affected (Polis & Strong 1996). Variation among Wiens 1996). communities in the commonness of intraguild preda- At Manitou, mistletoe is fed upon by three specialist tion has been proposed to be responsible, in part, for herbivores: caterpillars of Dasypyga alternosquamella variation in the strength of trophic cascades (Shurin Ragonot (Pyralidae, Lepidoptera) and Promylea et al. 2002). lunigerella glendella Dyar (Pyralidae, Lepidoptera), Considerably less attention has been given to the and the sap-feeding Neoborella tumida Knight potential for trophic structure to be variable within (Miridae, Hemiptera) (Mooney 2001; Mooney 2003). communities (but see Sipura 2002; Moon & Stiling A more diverse herbivore community feeds upon pine: 2004). Among-study variation in the impacts of pred- caterpillars (three species of Geometridae, and two ators has been attributed, at least in part, to character- from other unidentified families), leaf- and plant- istics of the herbivores and plants involved (Schmitz hoppers (Homoptera, suborder Auchenorrhyncha; et al. 2000; Halaj & Wise 2001). Furthermore, there are 36 species), and the aphid Cinara schwarzii Wilson reasons to predict within-community variability in (Aphididae, Homoptera). Cinara schwarzii is a facultative trophic cascades as a function of the specific character- mutualist with wood ants Formica spp. (Formicidae), istics of the plants and herbivores involved. Plants but the hoppers at this site are not. Mistletoe and pine influence rates of herbivore damage directly through tissues damaged by caterpillars are easily recognizable, constitutive (Fritz & Simms 1992) and induced but feeding by sap-feeders is not. Based on extensive (Karban & Baldwin 1997) resistance traits, and observations conducted with this community, we are indirectly via predators (Turlings, Tumlinson & Lewis certain that these herbivores are host-specific, and that 1990; Marquis & Whelan 1996). Likewise, herbivore no herbivore feeds upon both pine and mistletoe. characteristics such as concealed vs. exposed feeding Ninety-five per cent of bird foraging on pine and modes (e.g. Fritz 1983), sequestration of plant secondary mistletoe is performed by insectivorous chickadees compounds (e.g. Dyer & Bowers 1996) and predator Parus spp. (Paridae), nuthatches Sitta spp. (Sittidae) avoidance behaviours (Preisser, Bolnick & Benard and warblers Dendroica spp. (Parulidae) (Mooney, in 2005) influence rates of predation. press). The arthropod predator community is domin- We compared the effects of insectivorous birds ated by generalists that move freely between pine and between two sympatric plants, and between two guilds mistletoe (Mooney, in press), including ants, hunting of herbivores. For 3 years we excluded birds (top spiders and web-spinning spiders. Ladybird beetle larvae predators) from ponderosa pine Pinus ponderosa Laws. and adults (Coccinellidae), lacewings (Neuroptera, scopulorum and its angiosperm parasite the south- Chrysopidae), and some hemipterans are most com- western dwarf mistletoe Arceuthobium vaginatum (Willd.) monly associated with aphids, but also prey upon other Presl ssp. cryptopodum (Engelm.) Hawksw. & Wiens. arthropods opportunistically (Dixon 2000; Wheeler Pine and mistletoe are each fed upon by separate 2001). species of foliage-chewing and sap-feeding herbivores. These are in turn preyed upon by arthropods (interme-   diate predators) that move freely between the two plants. By studying a gymnosperm and a parasitic angiosperm, In late June 1999, 32 understorey pines were selected. we sought to increase the taxonomic and ecological Each was 1–3 m tall (mean ± 1 SE: 2·6 ± 0·07), and diversity of the plants for which bird effects on plant heavily parasitized by mistletoe. Ring counts of trunks growth have been measured. By comparing the effects showed them to be 71 ± 4 years old. Sixteen trees each of this single predator community between two her- were assigned to bird exclusion (cage) and control bivore guilds and between two sympatric plants, we treatments. Cages consisted of a frame of four metal sought to test whether trophic cascades vary within this bars (1·25 cm diameter) and a polyvinyl chloride community, and to associate the differing effects of plastic tubing roof wrapped in 2·5 cm mesh monofila- birds with characteristics of the herbivores and plants ment netting (Marquis & Whelan 1994). involved.     Materials and methods Visual searches were used to quantify the abundance of      pine herbivores and the arthropod predators that moved between both pine and mistletoe. Mistletoe-feeding © 2006 The Authors. This work was conducted at the Manitou Experimental herbivores were small and extremely cryptic in their Journal compilation Forest in Woodland Park, Colorado, U.S.A. (39°06′N, coloration, and mistletoe morphology made them © 2006 British 105°05′W) at an elevation of 2400 m. The field site difficult to detect. Consequently, data on these arthro- Ecological Society, Journal of Animal was in a pure stand of ponderosa pine parasitized by pods were not collected. Our understanding of bird Ecology, 75, south-western dwarf mistletoe. Dwarf mistletoes effects on mistletoe herbivores is thus limited to 350–357 Arceuthobium spp. (Viscaceae) are leafless, dioecious inference from data on herbivore damage to mistletoe

352 tissues (see below). At this site, mistletoe caterpillars unopened 2002 buds. The pre-experimental internodes, K. A. Mooney & were very common (as high as 2·5 per plant in 1999: which were free of foliage, were measured for the fresh Y. B. Linhart Mooney 2001, 2003) while N. tumida was relatively less mass of wood alone. For the 1999–2002 internodes, the common (0·3 per plant, Mooney unpublished data). fresh mass of foliage, wood and buds combined was Fourteen separate 12-min searches were conducted measured. The wood and bud lengths and diameters for arthropods over three summers: four in 1999 and from these experimental internodes were recorded to five in 2000 and 2001. This nondestructive method for calculate volume. Regression equations were derived measuring arthropod abundance has the advantage of from 100 nonexperimental branch tips relating wood not disturbing populations that are to be monitored for and bud volume to mass. Experimental wood and bud extended periods of time. Each search was limited to mass were then estimated for each branch section with pine branches and foliage. While observations were these equations (wood mass (g) = −0·027 + 0·0037 × wood recorded for predators foraging on mistletoe, no specific volume (mm3), R2 = 0·89; bud mass (g) = −0·0255 + effort was made to search mistletoe shoots. The 2·8 h of 0·0015 × bud volume (mm3), R2 = 0·92). Finally, this observation per tree did not constitute a census all arth- estimated wood and bud mass were subtracted from ropods, but provide an indication of relative arthropod the measured combined mass of wood, bud and foliage abundance on bird exclusion and control trees. to get an estimate for foliage mass alone. This procedure In each visual search, arthropods were counted as thus generated mass values for production of pre- belonging to seven categories: (1) ants; (2) aphids; experimental wood, experimental wood and experimental (3) aphid-specialist predators (coccinellids, neuropterans, foliage. Within each branch tip, wood and foliage growth Phytochoris spp. (Miridae, Hemiptera), Daerocoris spp. during experimental years were each significantly (Miridae, Hemiptera)); (4) pine-feeding caterpillars; correlated with pre-experimental wood growth (foliage, (5) hoppers; (6) hunting spiders (Salticidae, Anyphae- r = 0·64, n = 480, P < 0·0001; wood, r = 0·79, n = 480, nidae); and (7) web-spinning spiders (Theridiidae). P < 0·0001). Relative growth rates (‘RGR’) were calcu- Individual tree counts were low. Counts were summed lated for wood and foliage production by dividing across all trees and sampling periods within year and the mass produced during the experiment by pre- treatment for six totals per arthropod group (exclusion experimental wood growth. The mean wood RGR and and control for 1999, 2000 and 2001). Separate tests foliage RGR for each tree were the dependent variables were performed with χ2 for effects of: (1) bird exclusion in statistical tests for the effects of bird exclusion on across all years; (2) year per se, i.e. combining exclusion pine growth (see Estimating effect size below). and control trees; and (3) the interaction between year and bird exclusion (Zar 1999). In tests for year effects,    annual total arthropod counts for 2000 and 2001 were each multiplied by 0·8 to account for unequal sampling In September 1999 and June 2002 the total number of effort among years (i.e. a χ2 comparison of three values mistletoe plants on each pine and the proportion of for each arthropod group: Σ1999, Σ2000*0·8, Σ2001*0·8). those plants with signs of caterpillar herbivory were This transformation reduced the size of the counts upon recorded. For each tree the 2002 counts were divided by which the χ2 were calculated, thus lowering statistical 1999 counts and λ, the annual population growth rate, λ 1/3 power and making this a conservative test (Zar 1999). was calculated as = (n2002/n1999) (Hastings 1997). The table-wide false positive rate was controlled with a For each tree, the total proportion of mistletoe plants sharpened step-up Bonferroni (Garcia 2004). with herbivore damage was calculated for 1999 and 2002 combined. Per cent herbivory and λ were the two      dependent variables used in tests for the effects of bird exclusion on mistletoe (see Estimating effect size below). Herbivore damage on 40 pine needles produced in each year was measured in early June the year after the nee-    dles were produced. Needles were selected haphazardly across the entire canopy, and damaged needles were To test for the effect of bird exclusion on the five plant compared with other needles in their fascicles to esti- variables, effect sizes (Hedges, Gurevitch & Curtis mate original length. Percentage needle tissue damaged 1999; Osenberg, Sarnelle & Goldberg 1999) were was the dependent variable in statistical tests for the calculated as the natural log of the ratio of exclusion = effects of bird exclusion on herbivory (see Estimating mean to control mean ( ln(xexclusion/xcontrol)). Ninety- effect size below). five per cent confidence intervals (‘95% CI’) were calculated by bootstrapping with 999 replications © 2006 The Authors.     (Potvin & Roff 1993). Means and confidence intervals Journal compilation were back-transformed from log ratios to the propor- © 2006 British In June of 2002, 15 branch tips were collected from each tional change in each dependent variable caused by Ecological Society, = Journal of Animal tree and cut at the nodes to isolate the pre-experimental bird exclusion ( xexclusion/xcontrol). The proportions were Ecology, 75, wood and foliage (1997–98) from the wood and foliage multiplied by control tree mean to express effect sizes in 350–357 of the experimental internodes (1999–2001) and the original units.

353 Table 1. Statistical tests for effects of bird exclusion (cage), year and the interaction between the two on pine canopy arthropods Contrasting cascades from birds Cage Year Cage*Year χ2 χ2 χ2 Arthropod group ()df=1 PPP( df=2) ()df=5

Ants 461·52 0·0001 158·63 0·0001 118·06 0·0001 Aphids 251·45 0·0001 133·16 0·0001 56·82 0·0001 Caterpillars 0·13 0·72 1·34 0·52 3·10 0·68 Hoppers 2·15 0·15 11·78 0·0027 2·23 0·82 Aphid-specialist predators 6·91 0·0086 22·78 0·0001 9·00 0·11 Hunting spiders 9·57 0·0019 54·51 0·0001 2·46 0·78 Web-spinning spiders 0·99 0·32 17·33 0·0002 1·09 0·95

Note: Analyses performed on total counts of all trees within a treatment (n = 16 per treatment) across all sampling periods within a year (1999 = 4, 2000 = 5, 2001 = 5). For tests of year effect, the 2000 and 2001 counts were multiplied by 0·8 to equalize with the 1999 sampling effort. The table-wide false-positive rate is protected with a sharpened step-up Bonferroni adjustment, and significant results (corrected α = 0·0051) are shown in bold.

An effect whose 95% confidence interval does not original analysis (Fig. 1). Post-hoc χ2 analysis for 2001 include a zero effect is significant (P < 0·05). Failure to showed an effect of birds on aphid-specialist predators χ2 reject the null hypothesis can occur because (1) treat- (()1 = 15·6, P < 0·0001, 30% reduction) but not on χ2 ment effects are weak and of little biological relevance hoppers (()1 = 3·76, P = 0·0525). (the conclusive null result), or (2) the experiment has low statistical power and biologically relevant treatment       effects may or may not exist (the inconclusive null result) (Steidl, Hayes & Schauber 1997). The confidence Pine needle herbivory (mean ± 1 SE) across all trees intervals around nonsignificant effects provide an and years was 4·1 ± 0·3%. There was no bird effect estimate – with 95% certainty – of the maximum and (Fig. 2, ‘foliage damage’). Ninety-five per cent confid- minimum effects that could have been missed (Type II ence intervals for this null result did not include a level error) by the conducted experiment. The confidence of herbivory likely to be of biological significance to intervals of nonsignificant results can then be inter- pine, making this a conclusive null result (95% CI: preted in light of the biology of the system to make a +2·0% to −0·3% change in herbivory). There was no qualitative evaluation of whether the null result is con- detectable bird effect on foliage growth (Fig. 2), but clusive or inconclusive (Steidl et al. 1997). this null result was inconclusive because the large confidence interval (95% CI: +23% to −22% change in foliage RGR) meant this experiment had insufficient Results statistical power to address this question. Bird exclu-     sion significantly reduced wood RGR by 11% (95% CI: −21% to −2% change in wood RGR) (Fig. 2). The 5004 arthropods observed (66% on caged trees) were 41% ants, 16% tended aphids, 15% hunting spiders,    12% aphid-specialist predators, 9% web spinning spiders, 6% hoppers and 1% caterpillars. Excluding Averaged across 1999 and 2002 there were 65 ± 7 mis- birds with cages caused significant increases in ants, tletoe plants per host pine and 18 ± 1·4% of mistletoe tended aphids, and hunting spiders (Table 1, Fig. 1). plants had herbivore damage. There was no bird effect Bird effects on hunting spiders were constant among on caterpillar damage to mistletoe tissues (Fig. 2), and years (20% reduction) but there were interactions with the relatively narrow confidence interval makes this a year for effects on ants and tended aphids. Birds affected conclusive null result (95% CI: +5% to −7%). Averaged χ2 λ ± ants in each year (1999 ()1 = 42·7, P < 0·0001, 47% across both treatments, was 1·05 0·03 and unaffected χ2 reduction; 2000 ()1 = 30·1, P < 0·0001, 37% reduction; by bird exclusion (Fig. 2). This null result, presented as χ2 2001 ()1 = 480·4, P < 0·0001, 81% reduction). Aphid percentage change in mistletoe abundance per year abundance was lower in the presence of birds the later (= (λ − 1) * 100), was somewhat inconclusive given the χ2 2 years (2000 ()1 = 38·5, P < 0·0001, 56% reduction; range of possible effects that we may have failed to χ2 − 2001 ()1 = 247·6, P < 0·0001, 87% reduction) but not in detect (95% CI: +11% to 17%). © 2006 The Authors. χ2 the first year (1999 ()1 = 4·0, P = 0·047 not significant Journal compilation at adjusted α = 0·017). There was no effect of birds or © 2006 British Discussion bird-by-year interaction on aphid-specialist predators Ecological Society, Journal of Animal or hoppers, but in 2001 the patterns in the means The presence of birds in pine canopies was associated Ecology, 75, among the five sampling dates suggested the possibility with increased pine wood growth, and this was likely 350–357 of real treatment effects that were missed in our a direct result of birds having negative effects on 354 K. A. Mooney & Y. B. Linhart

Fig. 1. Arthropod abundance on bird exclusion (solid circles) and control trees (hollow circles) from 1999 to 2001. Predator data were collected from both pine and parasitic mistletoes. Data on herbivores are exclusively from pine. Mean (± 1 SE) arthropods per 12-minute search are shown on the Y-axis. Letters next to arthropod group in each panel indicate significant (P < adjusted α of 0·0051) effects of bird exclusion (‘B’), year (‘Y’) and their interaction (‘B*Y’). See Table 1 for detailed statistics. ‘†’ indicates a posthoc tests was conducted; bird exclusion affected aphid-specialist predators in 2001 (P < 0·0001) but not hoppers (P > 0·05).

ant-tended aphids in 2000 and 2001. Aphids have been exclusion on ant-tended aphids, there were not detectable shown to affect the wood growth of other tree species as effects on untended pine herbivores (hoppers and cat- well (Dixon 1971b,a). Aphid feeding has also been erpillars), nor were there indirect effects of bird exclusion shown to reduce foliage growth of trees in other set- on damage to pine foliage. As a result, we assume that tings (e.g. May & Carlyle 2003; see review in Dixon the mechanism for the indirect impact of birds on pine © 2006 The Authors. 1998), but the high variance of foliage mass in our growth was via direct effects on ant-tended aphids. Journal compilation experiment meant we had inadequate statistical power This stark difference in the effects of bird exclusion © 2006 British to resolve this question (Steidl et al. 1997). The varia- between ant-tended and untended pine herbivores Ecological Society, Journal of Animal bility we observed was likely because foliage mass was suggests there was within-community variation in food Ecology, 75, a function not only of growth, but also of subsequent web structure (Fig. 3). The negligible impact of bird 350–357 needle abscission. In contrast to the effects of bird exclusion on untended herbivores is consistent with 355 Contrasting cascades from birds

Fig. 2. Effects of birds on pine and mistletoe. Effect sizes (back-

transformed log[xexclusion/xcontrol]) were multiplied by control tree means to report effects (± 95% confidence interval) in original units. Pine foliage damage = % needle tissue damaged. Pine foliage and wood growth = % change in relative growth rates. Mistletoe plant damage = % change in the number Fig. 3. Proposed food web models for the variable top-down plants fed upon by caterpillars. Mistletoe population size = % effects of birds. Effects (+, –) are transmitted from the base to change per year in number of mistletoe plants per host tree the head of each arrow, and the arrow width represents λ − (= ( 1) * 100). Effects with confidence intervals spanning interaction strength. Solid-straight arrows are direct effects, zero are not significant. and dashed-curved arrows are indirect effects. Left diagram: effects of birds on pine via predation of ant-tended aphids (F. podzolica tending C. schwarzii). Bird feeding upon aphids compensatory predation (Polis & Strong 1996) by the indirectly increased pine growth. We do not include predatory hunting spiders, ants and aphid-specialist predators arthropods in this model because aphids were protected by that were more abundant in cages. Aphid-specialist mutualist ants. Right diagram: effects of birds on pine and mistletoe via predation on predatory arthropods (left to right: predators are most often associated with aphids, but hunting spiders, ants, aphid-specialist predators) and also prey upon other arthropods opportunistically untended herbivores of pine (top to bottom: hoppers, (Dixon 2000; Wheeler 2001). Evidence from other caterpillars) and mistletoe (top to bottom: N. tumida and the work at this field site documents that ants remove pine caterpillars D. alternosquamella and P. lunigerella). Birds had herbivores (Mooney & Tillberg 2005; Mooney, in no net effects on the untended pine and mistletoe herbivores, or on pine and mistletoe biomass, due to compensatory press), and work from other systems documents the predation of untended herbivores by ants, hunting spiders, abilities of hunting spiders to reduce insect herbivore and aphid-specialist predators. We do not include web- abundance (see review in Wise 1993). Web-spinning spinning spiders in this model because they did not increase in spiders did not respond to bird exclusion, and thus abundance on caged trees and thus did not contribute to probably did not contribute to compensatory preda- compensatory predation. tion within cages. Our results for untended herbivores thus support the notion that reticulate food webs cre- experiment, but our arthropod data suggest the effect ated by intraguild predation resulted in compensatory of our manipulation likely varied among years. While predation that decoupled the effects of top predators ants responded immediately to bird exclusion in 1999, from plant growth (Polis & Strong 1996). aphid populations only increased on caged trees the In contrast to the above results for untended herbiv- subsequent year (2000). In the third year (2001), both ores, aphid populations burgeoned in the absence of aphid and ant populations showed dramatic peaks birds on caged trees despite increases in hunting spiders within cages. One explanation for this pattern is that and aphid-specialist predators. Ants apparently pro- ants benefited from bird exclusion first (1999), and only tected pine-feeding aphids from these intermediate subsequently (2000) did ant-dependent aphids begin to arthropod predators but not from insectivorous birds. accumulate on caged trees. In the third year (2001), In this way, the ant–aphid mutualism simplified the aphid populations continued to increase and this trophic pathway between birds and pine into a three- attracted additional ants. While this is a coherent tiered, linear food chain as envisioned by early food interpretation of our data, there are other possible web theorists (Hairston et al. 1960). Under these explanations. Precipitation levels decreased over the conditions, we observed a trophic cascade. The food 3 years of the experiment (annual total 43·9 cm in 1999, web structure linking insectivorous birds and pine thus 32·1 cm in 2000, 26·1 cm in 2001), and aphid response © 2006 The Authors. varied between ant-tended herbivores (= linear food to bird exclusion may have only occurred on drought- Journal compilation web structure) and untended herbivores (= reticulate stressed trees. In any case, these multiyear dynamics © 2006 British food web structure) (Fig. 3): the former, but not the underscore the need for long-term studies to properly Ecological Society, Journal of Animal latter, transmitted bird effects to pine. characterize trophic dynamics on long-lived plants. Ecology, 75, We measured the accumulated impact of bird Caterpillar damage to mistletoe was unaffected by 350–357 exclusion on pine growth at the conclusion of the bird exclusion. This suggests that predatory arthropods 356 may have compensated for bird predation of mistletoe on the evolution of herbivore communities could thus K. A. Mooney & herbivores as we have argued they did for the untended set the stage upon which birds currently act. Y. B. Linhart herbivores of pine. We did not quantify the abundance Our study documents the differing effects of a single of herbivores on mistletoe plants so our understanding predator community between two guilds of herbivores of this food web is incomplete. Pine and mistletoe and two plants. This adds to a small but growing liter- share a single community of predatory arthropods, ature demonstrating that the trophic linkage between and the higher abundance of arthropod predators in terrestrial predators and plants varies not only among, pine canopies almost certainly had implications for but also within ecological communities (Sipura 2002; mistletoe herbivores. In a separate 30-day trial conducted Moon & Stiling 2004). The patterns we observed at this site, we found that pine canopy arthropod suggested these variable effects of birds were a result of predators reduced the survival of mistletoe-feeding differing food web structure, and these effects supported caterpillars by one-third (Mooney, unpublished data). current food web theory: linear food chains resulted The higher abundance of predatory arthropods on in trophic cascades, while the reticulate food webs and around mistletoe plants in cages thus provides an associated with intraguild predation did not. Our study explanation for why caterpillar damage to mistletoe adds to this understanding by demonstrating that var- and mistletoe population size did not respond to bird iation in food web structure occurs among sympatric exclusion. plants, and even among herbivores feeding on the same The pine and mistletoe systems were linked directly plant. by competition for resources between the two plants, and indirectly by a shared community of predatory Acknowledgements birds and arthropods. As a result, dynamics occurring on one plant likely had consequences for the other. One This work was supported by funds from the U.S. Forest scenario, in particular, seems quite plausible to us; the Service Rocky Mountain Research Station and the higher abundance of aphids on caged pines may have University of Colorado at Boulder. Brian Geils (U.S.D.A. recruited additional ants that then preyed upon Rocky Mountain Research Station) provided valuable mistletoe herbivores. We have observed that ants advice on mistletoe-related matters. Kathy Thomas tending pine-feeding aphids also preyed upon mistletoe and Ken Keefover-Ring helped in the field. M. Deane herbivores and reduced mistletoe damage (Mooney, Bowers advised the experimental design and execution. pers. ob). More generally, there is extensive evidence Comments from M.D.B., B.G., Sharon Collinge, Carl that both plants and herbivores interact indirectly via Bock, Michael Singer and the Plant–Animal Interac- shared enemies (Holt & Lawton 1994). By comparison, tion group at Boulder improved this manuscript. Wayne there is relatively less known about how plants interact Shepherd and Steve Tapia (Rocky Mountain Research indirectly via shared predator communities (but see Station) provided logistical assistance. Linhart et al. 2005). 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© 2006 The Authors. Journal compilation © 2006 British Ecological Society, Journal of Animal Ecology, 75, 350–357